I need to top off the R-12 in my A/C. I have the small 14 oz cans of R12, a top tapper and charging hose. I know to feed GAS into the low-side while watching the sight glass for bubble reduction. What I don't know is the proper way to hook up the charging hose. Do I tap the can, vent off a little R12 and quickly attach to the low side connection? Is there a better way to purge the charging line? I am trying to avoid the introduction of non-compressible gasses into the system.
Thanks.
got my 609 too :-)

Personally, I would never add freon without being able to monitor both high and low side pressures. If you get too much pressure in the high side hose it can explode, so it's a very good idea to know what's going on in your system. The method you describe is basically the method that I use with my manifold and gauges, but with the added step of bleeding two more lines. I like to thread the fitting into the low side service valve (not enough to seat it) before opening the valve on the can tap. That way I can seal the line quickly. Unless you're pulling a vacuum on the system, I don't know of a better way. However, this is an old-fashioned way of doing it, and someone may have since come up with a better way.

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1982 300D, black w/ palomino leather, 203K
2001 E320, champagne, 105K (Mom's)
1999 Dodge Dakota, squeaky & battered, 142K

No way to do it with just a can tapper.

With hoses and a set manifold guages, it works as follows. Close both hi & low valves. Attach both hi & low side hoses. Crack open the low side valve, allowing a slow flow of refrigerant up the low side hose, into the manifold, and down the charging hose to the can tapper. After a few seconds of allowing the refrigerant flow to push the air out of the hoses, tap the can.

There are other procedures as well.

Can't do this with your setup because you don't have a shutoff valve to control refrigerant flow though the hose.

- JimY

"I am trying to avoid the introduction of non-compressible gasses into the system."
An admirable Goal...

That starting the R12 through the line with it attached but on lightly....and then tightening it was the old style way of doing it...
I think they have new hoses ... which I don't have yet... which are designed to either avoid that or minimize the amout of R12 vented to accomplish that goal...
But I am surprised that that was not covered in the 609 test.. ???????

I agree that you are trying to do this with less tools than is required to do it properly or safely...
You can run your system many many ounces lower than the max charge with few bad results compared to even ONE ounce over charged.. that really works against the whole AC gas/liquid change in pressure/soak up heat theory...

Something else I have heard is an overcharge can bend, break or wear the piston vanes inside the compressor and possibly the shaft seal. especially with R134a. high side monitors the pressure comming out of the compressor and really alerts you of an over charge. Sight glass can be misleading.

Go/log on to Harbour Freight tools; Nice new R12 gauge sets $39, R134A sets $49.

That is good info about the hose purge, I wish my former indy tech did this ($100 quote for evac/charge with my R12and my gauges) Along with his upside down discharge can (liquid) into low side, shotty 15 min vacuum. I trusted this guy until i watched him and got educated here. Now he says i have a bad compressor, I think he did it but can not prove it, he was quick to reclaim 1.65 lbs of my R12 and says it is too contaiminated to reuse. btw I paid him nothing and wont go back.

DIY forever on everything is my new motto.

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1993 Mercedes Benz 300D 2.5 205K (ex wife's)
1984 Mercedes Benz 300SD 320K (SOLD)
2004 Mercedes Benz C240 75K
1995 GMC Sierra 2WD 5.7L 188K
1983 Mercedes Benz 300SD 239K (SOLD)
1987 BMW 325i 220K (SOLD FOR SALVAGE)

609 Certification

About 1972 I busted two rebuilt compressor in one day putting liquid refrigerant into the suction side of the compressor... PING went the reed valves... but I was young ... will let yall fill in the rest of the sentence....

For a 'pro' that guy is a real loser... you are on the right track....

I didn't find much of the information in the 609 study manual useful for actually working on A/C. You need to know what color cylinders different refrigerants are stored in, a bit about the ozone depletion theory, flammable refrigerants are against federal law in automotive A/Cs, cross-contamination of refrigerants is bad, etc. Nothing in the test actually tests ability to do A/C work, or knowledge of how A/C functions. Remember, we're dealing with the federal government here, so don't expect it to make too much sense

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1982 300D, black w/ palomino leather, 203K
2001 E320, champagne, 105K (Mom's)
1999 Dodge Dakota, squeaky & battered, 142K

Well, that makes sense... they are mainly charged with making sure AC guys know about the ozone deal.... and federal regulations....

Non-compressable ? ..I think all gases are compressable.....
You don't mean...non-condensable do you ?


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I think that is short hand in the industry for non compressible into liquid AT the Temperatures and Pressures available inside the AC system.....the working parameters of the compressor .....